Surface Decarburization-Restrained Steel and Manufacturing Method Thereof

ABSTRACT

Decarburization-restrained steel and manufacturing method thereof are disclosed. Steel includes a boron (B)-concentrated layer formed on its surface to prevent carbon of the steel from being in contact with oxygen in atmosphere to thus restrain decarburization of the steel. The steel includes a boron-concentrated layer with a thickness of 3 mm or larger formed on the surface of the steel. The method of manufacturing decarburization-restrained steel includes cooling steel containing 0.001 wt % to 0.02 wt % of boron (B) at a cooling speed of 0.5° C./s to 25° C./s at an austenite+ferrite two-phase region.

TECHNICAL FIELD

The present invention relates to surface decarburization-restrained steel and a manufacturing method thereof and, more particularly, to steel including a boron (B)-concentrated layer formed on its surface to prevent carbon contained in the steel from being in contact with oxygen in atmosphere to thus restrain decarburization of the steel, and a manufacturing method of the steel.

BACKGROUND ART

Recently, in order to effectively use limited fossil fuel energy and secure economic efficiency over soaring oil prices, steel is required to have a high level of strength. Providing steel with a high level of strength may lead to a reduction in the number of steel components required for the manufacture of diverse products and allow for this diversity of products to have advanced functions, potentially resulting in an improvement of mileage (fuel efficiency) in a product consuming large amounts of oil, such as automobile, to serve to greatly reduce energy costs.

Carbon is one of the most generally added elements in order to secure the strength of steel, and in particular, steel used for automobile components contains an amount of carbon greater than a certain predetermined level. However, in manufacturing structural high strength steel, generally used as a material of automobile components or the like, a surface layer of the steel is likely to be decarburized. That is, when an austenite tissue having a high level of carbon solubility is transformed into a ferrite tissue having little carbon solubility, the activity of carbon increases, and in this case, carbon having high activity is brought into contact with an oxidation atmosphere on the surface of steel so as to react and cause the decarburization phenomenon. In other words, the decarburization phenomenon frequently occurs when the material is thermally treated at a high temperature range in which carbon can spread sufficiently, thus making carbon elements present within the steel quickly move to the surface layer of the steel through spreading and then being brought into contact with oxidative gas in the atmosphere. The presence of a surface-defective tissue represented by a decarburized tissue resulting from decarburization occurring from the surface layer of the material in manufacturing the structural high strength steel cause various problems. Namely, the decarburized tissue present on the surface has a considerably low hardness and corrosion resistance compared with the base material, so the steel with the decarburized tissue is degraded in its function, which is inadequate to be used as structural high strength steel. Namely, if structural high strength steel, facing continuously repeated fatigue, has such surface decarburized layer, the surface decarburized layer acts as a starting point for the generation and propagation of cracks, leading to crucial problems such as a shortening of the life span of the material, namely, a rapid failure of the material after manufacture, and the like.

Thus, a deliberate failure to maintain the material in a high temperature range may be considered as a method to restrain decarburization in manufacturing steel. However, the material must necessarily be subject to a high temperature manufacturing process such as rolling or forging in order to control the shape of metal and secure the quality of the material, so that the evasion of decarburization reaction is never easy and effective restraint of decarburization is a key point.

In addition, because a high level of strength in the material is to be attained within the bounds of minimizing the increase in a unit cost, in most cases, silicon (Si), a low-priced alloy element, is added to structural high strength steel in the related art. However, silicon (Si), a fourth group element, like carbon (C), having a similar behavior to that of carbon (C), not only increases the activity of carbon (C) but also serves to stabilize a ferrite region, so ferrite is easily formed to reduce the solubility of carbon (C). Namely, the addition of silicon (Si) extends a so-called two-phase region in which austenite and ferrite coexist, and in this case, escaping carbon from the surface layer of the steel surface portion due to oxidation is accelerated so as to pass through the large two-phase region for an extended period of time, promoting the decarburization reaction.

In an effort to solve the problem, a control cooling pattern for quickly evading the vicinity of the two-phase region where decarburization is intensified has been introduced to be used. However, it is difficult to control a temperature deviation or the like in a coil due to the difference in a piling density in cooling the material, and in particular, the temperature deviation becomes severe in coil overlapping portions and the like, making it difficult to completely restrain partial spurring decarburization. In addition, in the case of high silicon steel containing a large amount of silicon for high strength, the temperature section of the two-phase region is greatly extended compared with general steel, which is only avoided through a controlled cooling speed, so it is therefore urgent to develop steel whose decarburization sensitivity is basically suppressed.

To this end, a method in which scale generation elements such as chromium (Cr), copper (Cu), nickel (Ni), or the like, are added to steel to increase the density of a scale inevitably generated during heating at a high temperature and improve adhesion with the base material to thereby prevent contact between the carbon element of the surface layer of steel and oxidative gas, or the like, has been studied. However, this method has a problem in that the adhesion between the base material and the scale is reduced, failing to completely restrain the decarburization reaction.

As stated above, steel, whose surface can be effectively restrained from decarburization in manufacturing, is yet to be developed.

DISCLOSURE OF INVENTION Technical Problem

An aspect of the present invention provides steel including a dense concentrated layer formed on its surface to prevent carbon of the steel from being brought into contact with oxidation atmosphere.

Solution to Problem

According to an aspect of the present invention, there is provided steel including a boron-concentrated layer with a thickness of 3 mm or larger formed on the surface of the steel.

The boron-concentrated layer may be a region where the content of boron is 10 times or larger the average composition of steel.

The steel may contain 0.001 wt % to 0.02 wt % of boron.

The steel may further contain 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti), in addition to the boron component.

The steel may further contain 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S).

In addition to the components, the steel may further contain 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb).

According to another aspect of the present invention, there is provided a method of manufacturing decarburization-restrained steel, including: cooling steel containing 0.001 wt % to 0.002 wt % of boron (B) at a cooling speed of 0.5° C./s to 25° C./s at an austenite+ferrite two-phase region.

In addition to the component, the steel may further contain 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti).

The steel may further contain 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S).

In addition to the components, the steel may further contain 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb).

ADVANTAGEOUS EFFECTS OF INVENTION

According to exemplary embodiments of the invention, because the boron-concentrated layer is formed on the surface of steel, carbon of the steel can be prevented from being in contact with oxidative gas in atmosphere, thus effectively restraining decarburization.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is PTA photographs confirming the results of the formation of a B-concentrated layer in a comparative example 4, an exemplary embodiment 3, and an exemplary embodiment 4.

FIG. 2 is a microscopic photograph showing the relationship among a boron-concentrated layer, a low-temperature tissue, and a ferrite decarburization layer based on cooling speed.

BEST MODE FOR CARRYING OUT THE INVENTION

The exemplary embodiments of the present invention will now be described in detail.

The inventors of this application devised the present invention upon recognition that, in order to prevent the formation of a decarburized layer on the surface of steel, a carbon component of the steel must be prevented from being in contact with oxidative gas in the atmosphere. In particular, the inventors discovered that a boron component is effective to prevent the carbon component of the steel from being in contact with oxidative gas in atmosphere.

Namely, because the boron component forms a dense concentrated layer on the surface layer of the steel, it can effectively prevent the carbon component from being in contact with oxidative gas in atmosphere. In particular, compared with the conventional scale formation elements such as chromium (Cr), copper (Cu), nickel (Ni), etc., even a small boron content obtains a sufficient effect and is effective for preventing decarburization. In an exemplary embodiment of the present invention, a boron-concentrated layer refers to a region formed on a surface layer of a wire rod where the content of boron (B) is larger than that of other regions. The boron-concentrated layer mainly includes a scale layer positioned at the outermost portion of the wire rod, and may be formed with a certain depth of the interior of the wire rod directly below the scale layer according to circumstances. Thus, in an exemplary embodiment of the present invention, the boron-concentrated layer is defined as a region of the surface layer where the content of boron (B) is ten or more times the average composition of steel to a certain depth.

According to the results of research conducted by the inventors of the present invention, the boron-concentrated layer must have a thickness of 3 mm or larger. If the boron-concentrated layer is too thin, it would easily segment, which could not properly hinder an infiltration of oxygen atoms, failing to accomplish the purpose of the present invention. Because the boron-concentrated layer usefully serves to restrain carbon from being in contact with oxidative gas, an upper limit of the thickness of the boron-concentrated layer does not need to be particularly limited. In this case, however, in consideration of the general content of boron (B), the boron-concentrated layer can be hardly formed with a thickness in excess of 120 mm.

Thus, the steel according to an exemplary embodiment of the present invention includes a boron-concentrated layer with a thickness of 3 mm or larger formed on the surface of the steel. The boron-concentrated layer is not only advantageous for restraining decarburization that may occur during a follow-up heating and cooling process but has already advantageously served to restrain decarburization during a manufacturing process of the steel. Namely, the concentrated layer is formed at a temperature range or higher at which the steel is easily decarburized, and restrains the decarburization of the steel during a cooling operation.

In addition, any steel is considered to belong to the category of the steel provided according to an exemplary embodiment of the present invention, so long as its surface decarburization is restrained, so there is no need to particularly limit other components. However, in order to form the boron-concentrated layer, a sufficient amount of a boron component must be contained in steel, so the content of boron is preferably limited to a certain range as follows:

Boron (B): 0.001 wt % to 0.02 wt %

As mentioned above, because boron (B) is the source of the boron-concentrated layer, it must be contained at 0.001 wt % or more. However, besides the role of forming the concentrated layer, boron serves to improve the hardenability of the steel. The steel for machine structural use according to an exemplary embodiment of the present invention needs to be soft to have sufficient processability before performing forging or heading process. Thus, if boron (B) is contained in excess, although the steel is manufactured through annealing (i.e., slow cooling), a great deal of low temperature tissues such as bainite or martensite are disadvantageously formed within the steel. Thus, in an exemplary embodiment of the present invention, the upper limit of the content of boron (B) is set as 0.02 wt %.

The effect intended by the present invention can be obtained by properly controlling the content of boron (B). Thus, preferably, the steel according to an exemplary embodiment of the present invention includes a boron-concentrated layer having a thickness of 3 mm or larger, for which boron (B) is contained by 0.001 wt % to 0.02 wt %.

When boron reacts with the nitrogen component present in the steel, precipitation such as BN or the like is generated. The precipitation serves to fix boron, preventing boron from spreading on the surface of the steel. Namely, the precipitation is an obstacle to the formation of the boron-concentrated layer. Thus, preferably, boron (B) exists in a state of so-called valid boron (or free boron) not fixed by nitrogen (N). To this end, it is effective for titanium (Ti) and nitrogen (N) to be controlled to be within the following ranges for the following reasons.

Nitrogen (N): 0.02 wt % or less

Nitrogen (N) is a gas contained by the atmosphere in large amounts, accounting for approximately 80% of the atmosphere. When nitrogen (N) is in contact with molten steel, it can be contained in large amounts by the molten steel. When nitrogen (N) is contained in steel, it reacts with boron present in the steel to form BN, and in this case, the spreading of boron (B) is restrained and boron (B) is fixed, so it is difficult to form a concentrated layer on the surface of the steel. Thus, the content of nitrogen (N) needs to be limited. In consideration of a processing load such as a steelmaking process or the like, the content of nitrogen (N) is limited to below 0.02 wt %. Preferably, nitrogen (N) is limited to below 0.01 wt %.

Titanium (Ti): 0.005 wt % to 0.5 wt %

As described above, nitrogen (N) is preferably restrained as little as possible, but cannot be completely removed in consideration of the processing load. Thus, in order to minimize the bad influence of nitrogen (N) inevitably contained in steel, titanium (Ti) is preferably added. Namely, titanium (Ti) is able to form nitrides (TiN, Ti(C,N), etc.), preceding boron, so it can restrain the formation of BN to allow more valid boron to remain. In consideration of the amount of nitrogen contained in steel, 0.005 wt % or more of titanium (Ti) must be contained to effectively secure valid boron (B). Titanium may sufficiently play its role by the amount up to 0.5 wt %, and an excess of more than the amount of titanium (Ti) would cause an increase in the unit fabrication cost or the like.

Thus, according to an exemplary embodiment of the present invention, steel in which the thickness of the boron-concentrated layer and the content of boron are controlled (adjusted) and titanium (Ti) and nitrogen (N) are contained at the afore-mentioned rates can be provided. Thus, with such conditions met, the advantageous effect of the present invention is useful regardless of the other remaining components of the steel.

Non-limited examples with respect to the composition of steel suitable for accomplishing the advantageous effect of the present invention will now be described to embody the present invention. Namely, an example of a composition of steel suitable for obtaining the effect of the present invention may be a composition in which boron (B), titanium (Ti), and nitrogen (N) are controlled to be within the above-mentioned ranges and carbon (C), silicon (Si), manganese (Mn), aluminum (Al), phosphorus (P), sulfur (S), etc., the basic components constituting the steel, are contained by the ranges as follows.

Carbon (C): 0.2 wt % to 1.0 wt %

Carbon (C) is an element added to secure the strength of the high strength steel. If the content of carbon (C) is less than 0.2 wt %, strength sufficient to ensure that the steel has the required level of strength is not realized. Conversely, if the content of carbon (C) exceeds 1.0 wt %, a proeutectoid cementite tissue is formed along grain boundaries to cause a material crack, significantly degrading fatigue strength. In addition, because it is difficult to secure sufficient toughness according to the high strength and restrain a material decarburization caused by an addition of silicon (Si), the content of carbon (C) is preferably limited to be 0.2 wt % to 1.0 wt %.

Silicon (Si): 0.1 wt % to 3.5 wt %

Silicon (Si) is employed in ferrite to have an effect of strengthening the strength of a base material. If the content of silicon (Si) is less than 0.1 wt %, it cannot exert the effect of strengthening the base material upon its being employed in ferrite. Thus, a lower limit of silicon (Si) needs to be limited to 0.1 wt %. Meanwhile, if the content of silicon (Si) exceeds 3.5 wt %, a possibility of generating center segregations increases and the activity of carbon is increased in thermal treatment to encourage surface decarburization. Thus, the content of silicon (Si) is preferably limited to 0.1 wt % to 3.5 wt %.

Manganese (Mn): 0.3 wt % to 1.0 wt %

Manganese (Mn) is an element useful for securing the strength of steel when it is contained in the steel. Thus, if the content of manganese (Mn) is less than 0.3 wt %, strength sufficient to ensure that the steel has the required level of strength is not realized. Conversely, if the content of manganese (Mn) exceeds 1.0 wt %, the toughness deteriorates. Thus, the content of manganese (Mn) is preferably limited to 0.3 wt % to 1.0 wt %.

Aluminum (Al): 0.1 wt % or less

The addition of aluminum (Al) reduces grain size and improves toughness. However, the content of aluminum (Al) exceeds 0.1 wt %, the amount of generated oxide-based precipitation increases and the size is coarsened, negatively affecting fatigue characteristics. In an exemplary embodiment of the present invention, aluminum is not an essential component of steel, so a lower limit of aluminum (Al) content is not set.

Phosphorus (P): 0.02 wt % or less

Phosphorus (P) segregates in a grain boundary to degrade toughness, so, preferably, it is not contained as much as possible. However, in order to completely remove phosphorus contained in steel from various sources, a huge load should be inevitably applied to a refining (or tempering) process. Thus, phosphorus may be permitted by the level of 0.02 wt % in which the phosphorus does not cause a big problem. If the refining load is not heavy, phosphorus is preferably limited to 0.01 wt % or less.

Sulfur (S): 0.02 wt % or less

Sulfur (S), a low melting point element, grain-boundary-segregates to degrade toughness and generates emulsions to negatively affect the characteristics of high strength steel. As such, preferably, sulfur (S) is not contained as much as possible, but in consideration of a load in a refining process, an upper limit of sulfur (S) is limited to be 0.02 wt %.

Accordingly, an example of a composition of steel for accomplishing an advantageous effect of the present invention, is a composition including 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S), in addition to the controlling of the thickness of the boron-concentrated layer to be within the above-mentioned range and including the elements such as boron (B), titanium (Ti), and nitrogen (N) within appropriate ranges.

Besides the above-mentioned components, an example of the composition of steel suitable for the present invention may include one or more of chromium (Cr), nickel (Ni), copper (Cu), oxygen (O), vanadium (V), niobium (Nb), and the like, within the following ranges in order to secure the physical properties of steel.

Chromium (Cr): 0.1 wt % to 1.5 wt %

Chromium is an element useful to prevent surface decarburization and obtain oxidation resistance and tempering and softening characteristics. If the content of chromium (Cr) is less than 0.1 wt %, it is difficult to obtain the sufficient oxidation resistance, tempering and softening characteristics, the surface decarburization preventing effect, and the like. Meanwhile, if the content of chromium (Cr) exceeds 1.5 wt %, deformation resistance deteriorates to potentially lead to degradation of strength. Thus, preferably, the content of chromium is 0.1 wt % to 1.5 wt %.

Nickel (Ni): 0.01 wt % to 1.0 wt %

Nickel (Ni) is an element added to effectively restrain surface decarburization by forming a scale layer with a high adhesion with the base material. Also, nickel (Ni) can effectively improve the toughness of steel. If the content of nickel (Ni) is less than 0.01 wt %, the effect of nickel (Ni) is not sufficient, while if the content of nickel (Ni) is 1.0 wt % or more, the amount of remnant austenite increases to reduce fatigue life. Also, because nickel (Ni) is high-priced, it causes a sharp increase in the unit fabrication cost. Thus, the content of nickel (Ni) needs to be limited to 0.1 wt % to 1.0 wt %.

Copper (Cu): 0.01 wt % to 1.0 wt %

An addition of copper (Cu) generates a scale layer with a high adhesion with the base material along with nickel (Ni), which is effective for preventing decarburization and improves corrosion resistance. This effect is reduced when the content of copper (Cu) is less than 0.01 wt %. Meanwhile, if content of copper (Cu) exceeds 1.0 wt %, it causes defective rolling by embrittlement.

Oxygen (O): 0.0020 wt % or less

Oxygen (O) is limited to 0.0020 wt % or less. If the content of oxygen (O) exceeds 0.0020 wt %, oxide-based nonmetallic inclusion is coarsely formed to sharply degrade a fatigue life.

Vanadium (V): 0.005 wt % to 0.5 wt %, and niobium (Nb): 0.005 wt % to 0.5 wt %

Vanadium (V) and niobium (Nb) are solely added or added together to form carbide/nitride to cause precipitation hardening to thus improve the strength characteristics of high strength steel. The contents of vanadium (V) and niobium (Nb) are limited to 0.005 wt % to 0.5 wt %, respectively. If the contents of vanadium (V) and niobium (Nb) are small, precipitation of vanadium (V) and niobium (Nb)-based carbide/nitride is reduced to obtain an insufficient improvement effect of grain boundary strengthening and fatigue characteristics. If the contents of vanadium (V) and niobium (Nb) are large, the unit fabrication cost rises sharply, a spring characteristics improvement effect by precipitation is saturated, and the amount of coarse alloy carbide not dissolved in the base material during an austenite thermal treatment increases to work as a nonmetallic inclusion to degrade the fatigue characteristics and precipitation strengthening effect.

The steel under the above-described conditions may be manufactured by a conventional hot rolling method. However, the present invention provides an example of manufacturing steel more effectively. An effective steel manufacturing method will now be described.

Cooling condition: An austenite+ferrite two-phase region is cooled at a cooling speed of 0.5° C./s to 25° C./s

As stated above, decarburization reaction occurs frequently from a two-phase region where temperature is high and at which ferrite starts to be generated. Thus, the boron-concentrated layer that can prevent the occurrence of decarburization at the temperature range should be generated to restrain decarburization. The research results of the inventors of the present invention show that, in order to obtain the above-mentioned effect, the boron-concentrated layer should be formed with more than a certain thickness without any segment, for which, thus, cooling needs to be performed at a cooling speed of 0.5° C./s or faster. If cooling is performed at below the cooling speed, even if the boron-concentrated layer is formed, its thickness may be too thin to be segmented, and the adhesion with the base material is not sufficient, so it is easily detached from the base material. Conversely, if the cooling speed is high, the boron-concentrated layer is formed with a sufficient thickness but a low temperature tissue is formed within the steel at the high cooling speed, which is not desirous for a follow-up process of the steel. This is because an area fraction of bainite or martensite, low-temperature tissues, of the fine tissues of the steel is preferably 5% or less. Thus, an upper limit of the cooling speed is set as 25° C./s. More preferably, the upper limit of the cooling speed is set as 20° C./s.

A general steel manufacturing method may be employed to manufacture steel before or after the cooling process. Namely, it is noted and understood that the hot rolling process for controlling the shape of steel by depressing the steel, or any other hot rolling process or an additional thermal treatment process after cooling operation, and the like, can be easily selected to be added by the skilled person in the art to which the present invention pertains.

MODE FOR THE INVENTION

Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Accordingly, the embodiments of the present invention are not limited to the above-described embodiments, but are defined by the claims which follow, along with their full scope of equivalents.

Exemplary embodiment

Individual batches of steel, each having the composition of Table 1 shown below (content of each component in Table 1 is expressed by weight percent (wt %) (except for nitrogen (N) and oxygen (O) which are expressed by ppm)), were cast to manufacture a billet (i.e., blooms), which were rolled at a depressing rate of 60% or larger in wire-rod rolling at a rolling termination temperature of 700° C. to 1,000° C., and then cooled at room temperature through various cooling patterns as depicted in Table 2 shown below to manufacture steel. The fine tissue of the steel manufactured under such conditions was observed. In this case, in order to quantitatively measure a low-temperature tissue and a decarburized layer depth, random observation was performed from thirty visual fields for the sake of convenience. The detailed values of the measured low-temperature tissue fraction and an average ferrite decarburized layer depth are shown in table 2.

The ferrite decarburized layer was measured from more than five visual fields from the carbon section of the wire rod and determined with the average value.

Also, in order to observe a scale layer by boron of the wire rod manufactured under the conditions of Table 1, a particle tracking auto-radiography (PTA) method, a boron distribution analysis method through neutron irradiation, was used for testing. This testing method is that when a thermal neutron is irradiated to a boron-contained material, heavy ions generated through a nuclear reaction are light-exposed to a film tightly attached to the surface of a test sample. Employing this method, whether or not the boron-concentrated layer was segmented and an average thickness of the boron-concentrated layer under the respective chemical components and cooling conditions were measured, and detailed results are shown in Table 2 and FIGS. 1 and 2. In this case, the low-temperature tissue fraction in Table 2 refers to the sum of area fractions of the bainite and martensite.

TABLE 1 Classification C Si Mn Ni Cr V Ti Cu Nb B P S Al N O Comparative 0.55 1.5 0.7 — 0.7 — — — — — 0.01 0.03 0.001 50 16 example 1 Comparative 0.40 2.2 0.7 — 1.0 0.1 — — 0.007 — 0.008 0.008 0.01 49 16 example 2 Comparative 0.50 2.3 0.7 0.3 1.0 0.2 0.03 0.4 0.008 — 0.009 0.007 0.06 55 14 example 3 Comparative 0.48 2.0 0.7 0.4 0.8 0.1 0.02 0.3 — 0.002 0.008 0.009 0.03 49 18 example 4 Comparative 0.52 2.1 0.5 0.5 1.0 0.10 0.02 0.5 — 0.003 0.009 0.015 0.05 53 17 example 5 Exemplary 0.53 1.5 0.7 — 0.7 — 0.02 — — 0.002 0.008 0.009 0.03 49 18 embodiment 1 Exemplary 0.52 2.1 0.5 0.5 1.0 0.10 0.02 0.5 0.01 0.003 0.009 0.015 0.05 53 17 embodiment 2 Exemplary 0.53 2.4 0.7 0.3 0.7 0.13 0.05 0.4 0.007 0.006 0.015 0.009 0.06 52 17 embodiment 3 Exemplary 0.55 2.2 0.7 0.4 1.0 0.2 0.07 0.4 0.02 0.005 0.007 0.006 0.06 50 15 embodiment 4

TABLE 2 Quality yield Presence or Low- Average Ferrite de- Average absence of temperature Heating cooling carburized thickness of boron scale tissue temperature, speed, layer depth, boron scale layer fraction, Classification ° C. ° C./s mm layer, μm fragment % Comparative 1350 1 0.05 — — 0 example 1 Comparative 1050 3 0.06 — — 0 example 2 Comparative 1100 5 0.04 — — 0 example 3 Comparative 1200 0.3 0.03 0.2 Fragment 0 example 4 Comparative 1350 30 0 70 Fragment 82 example 5 absence Exemplary 1350 0.5 0 98 Fragment 0 embodiment absence 1 Exemplary 1350 10 0 120 Fragment 0 embodiment absence 2 Exemplary 1100 3 0 3 Fragment 0 embodiment absence 3 Exemplary 1000 5 0 23 Fragment 0 embodiment absence 4

As noted in Table 1 above, comparative examples 1 to 3 represent compositions of individual steel under conditions in which a boron-concentrated layer could only be formed with difficulty because of the absence of boron. Comparative examples 4 and 5 and the exemplary embodiments represent each steel satisfying the advantageous compositions proposed in the present invention. In this case, however, as noted in Table 2, in comparative examples 4 and 5, the cooling speeds after heating are not within the desirous range defined by the present invention.

As noted from the results of Table 2, compared with the boron additive, in comparative examples 1 to 3, because boron was not added, a boron-concentrated layer was not formed during a cooling operation. Thus, the restraining of ferrite decarburization was not effective, resulting in the generation of ferrite decarburization from the surface of the ferrite steel to a deeper region. In particular, in comparative example 3, the components such as copper (Cu), nickel (Ni), and the like, were added like the related art to form a concentrated layer by the components. But it does not obtain the effect of preventing the occurrence of ferrite decarburization. Also, in comparative example 4, the composition of the steel satisfies the range of the present invention; however, the cooling speed was as low as 0.3° C./s. Because cooling was performed slowly, the boron-concentrated layer was thin, adhesion of a scale layer with a base material was reduced, and fragmentation occurs here and there, making an oxidative gas element and carbon come into contact through the gap between the fragments to promote decarburization. In case of comparative example 5 in which cooling was performed at a cooling speed of 30° C./s, a boron-concentrated layer of about 70 mm was formed so as to be greatly effective for preventing decarburization. However, the area fraction of a low-temperature tissue undesirably reached 82%.

In comparison, in the case of the exemplary embodiments 1 to 4 in which boron was added by 20 ppm to 60 ppm and the cooling conditions satisfied the range defined by the present invention, as confirmed in FIG. 1 and Table 2 (the drawing shows only the results of the exemplary embodiments 3 and 4), the boron scale layer was observed without any fragmentation and an average thickness of the scale layer was 3 mm to 120 mm. In addition, there is shown to be little decarburized layer, confirming that ferrite decarburization was significantly restrained by the boron-concentrated layer.

FIG. 2 shows microscope photographs obtained by observing whether or not ferrite decarburized layer was generated according to the formation of the boron-concentrated layer. As illustrated, in the case of comparative example 3 in which a concentrated layer is not formed, it is noted that a ferrite decarburized layer of about 50 mm was plainly formed on the surface. In case of comparative example 5, a boron-concentrated layer of about 70 mm almost prevented the occurrence of decarburization, but it is noted that a large quantity of low-temperature tissues such as martensite were formed. In comparison, however, in case of the exemplary embodiment 4 manufactured under the conditions of the present invention, ferrite decarburization was prevented and few low-temperature tissue was formed, confirming that the steel can be suitably used for post-processing.

Therefore, the advantageous conditions of the present invention can be ascertained. 

1. Surface decarburization-restrained steel comprising a boron-concentrated layer with a thickness of 3 mm or larger formed on the surface of the steel.
 2. The steel of claim 1, wherein the boron-concentrated layer is a region where the content of boron is 10 times or larger the average composition of steel.
 3. The steel of claim 1, wherein the steel contains 0.001 wt % to 0.02 wt % of boron.
 4. The steel of claim 3, wherein the steel further contains 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti).
 5. The steel of claim 4, wherein the steel further contains 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S).
 6. The steel of claim 5, wherein the steel further contains 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb).
 7. A method of manufacturing surface decarburization-restrained steel, the method comprising: cooling steel containing 0.001 wt % to 0.02 wt % of boron (B) at a cooling speed of 0.5° C./s to 25° C./s at an austenite+ferrite two-phase region.
 8. The method of claim 7, wherein the steel further contains 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti).
 9. The method of claim 8, wherein the steel further contains 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S).
 10. The method of claim 9, wherein the steel further contains 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb).
 11. The steel of claim 2, wherein the steel contains 0.001 wt % to 0.02 wt % of boron.
 12. The steel of claim 11, wherein the steel further contains 0.02 wt % or less of nitrogen (N) and 0.005 wt % to 0.5 wt % of titanium (Ti).
 13. The steel of claim 12, wherein the steel further contains 0.2 wt % to 1.0 wt % of carbon (C), 0.1 wt % to 3.5 wt % of silicon (Si), 0.3 wt % to 1.0 wt % of manganese (Mn), 0.1 wt % or less of aluminum (Al), 0.02 wt % or less of phosphorus (P), and 0.02 wt % or less of sulfur (S).
 14. The steel of claim 13, wherein the steel further contains 0.1 wt % to 1.5 wt % of chromium (Cr), 0.01 wt % to 1.0 wt % of nickel (Ni), 0.01 wt % to 1.0 wt % of copper (Cu), 0.0020 wt % or less of oxygen (O), 0.005 wt % to 0.5 wt % of vanadium (V), and 0.005 wt % to 0.5 wt % of niobium (Nb). 